Maternal Age-Related Risks for Down Syndrome and Other Trisomies

Risk for Trisomy 21 and Other Trisomies at Delivery

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The most common risk factor for Down syndrome is maternal age. As a woman gets older, her risk to have a pregnancy with a chromosome abnormality increases. Maternal age risks are often presented in chart format such as the chart below. In order to really understand the risk figures presented in this chart, it is important to understand how the data was collected and what it is actually telling you.

Why Do Maternal Age-Related Risk Charts Vary?

You may have seen different versions of maternal age-related risk charts and you may have noticed that there are differences in the risks numbers presented. There are a couple of reasons for these variations. Some charts present the maternal age-related risk for all chromosome abnormalities rather than age-related risk for Down syndrome. This chart presents both pieces of information. The middle column below shows the age-related risk of having a child with Down syndrome (trisomy 21) and the right-hand column shows the age-related risk for all chromosome related abnormalities (trisomy 21 and other trisomies - 13, 18, X and Y).

Maternal Age-Related Risks at Birth and at the Time of Prenatal Diagnosis

One other factor that accounts for some of the differences seen in age-related risk charts is that some charts present the chance of having a child with Down syndrome at birth (full term), and other charts present the chance at the time of prenatal diagnosis (early second trimester).

Because up to 25% of pregnancies with Down syndrome are miscarried or stillborn, and over 70% of pregnancies with trisomy 13 and trisomy 18 are lost before birth, the age-related risks at the time of prenatal diagnosis are higher than is the age-related risk at birth. The chart below presents maternal age-related risks at birth.

Age at the Time of Conception Compared With Age at Delivery

Many women wonder why we talk about the age of an egg at conception but use an age-related risk based upon your age at the time of delivery. This is because of how the studies were done to collect this data. Most of the data came from looking at the diagnosis rate of infants at the time of delivery. Because of premature births and miscarriages, it was not possible to accurately go backward nine months and come up with a maternal age conception-related risk to have a child with a chromosome abnormality. The chart below gives you your risk to have a child with a chromosome abnormality based on your age at the time of delivery.

What This Maternal Age-Related Risk Chart Tells You

This chart tells what your chance of having a child with Down syndrome, and your chance to have a child with any chromosome abnormality is, based on a mother’s age at the time of delivery. For example, if you (the mother) will be 39 years old at the time of delivery, the chance to have a child diagnosed with Down syndrome at birth is 1 in 137 (a little less than 1%), the chance to have a child diagnosed with any chromosome abnormality (including Down syndrome) at birth is 1 in 83 (or slightly higher than 1%).

Who Should Not Use This Chart?

If you have had a previous child or pregnancy with trisomy 21 or another trisomy, you risk may be different than the age-related risks presented in this chart. If you have a family history of a chromosome translocation or a previous child with translocation Down syndrome, you should consult with a genetic counselor who can give you more specific information about your risk depending on the type of translocation and your chromosome results. If you have conceived a pregnancy using a donor egg, you should use the age of the woman that donated the egg at delivery. In all of these situations, it is best to talk to a genetic counselor to get an accurate assessment of your risks.

The chart is below:

Maternal Age Related Risks

Mom's Age                     Risk for trisomy 21 (Down syndrome)                  Risk for all triomies
201 in 1,6671 in 526
211 in 1,4291 in 526
221 in 1,4291 in 500
231 in 1,4291 in 500
241 in 1,2501 in 476
251 in 1,2501 in 476
261 in 1,1761 in 476
271 in 1,1111 in 455
281 in 1,0531 in 435
291 in 1,0001 in 417
301 in 9521 in 384
311 in 9091 in 384
321 in 7691 in 323
331 in 6251 in 286
341 in 5001 in 238
351 in 3851 in 192
361 in 2941 in 156
371 in 2271 in 127
381 in 1751 in 102
391 in 1371 in 83
401 in 1061 in 66
411 in 821 in 53
421 in 641 in 42
431 in 501 in 33
441 in 381 in 26
451 in 301 in 21
461 in 231 in 16
471 in 181 in 13
481 in 141 in 10
491 in 11

1 in 8

Sources:

Hook EB, Cross PK, Schreinemachers DM. Chromosomal abnormality rates at amniocentesis and in live-born infants. JAMA 1983;249(15):2034-38.

Newberger, D., Down Syndrome: Prenatal Risk Assessment and Diagnosis. American Family Physician. 2001.

Down syndrome births in the United States from 1989 to 2001. Egan JF - Am J Obstet Gynecol - 01-SEP-2004; 191(3): 1044-8.

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